1. Field of the Invention
The present invention generally relates to a communication system for performing an equalization process, and more particularly to a communication system for effectively employing a forward equalization process and a backward equalization process and a communication system for realizing better convergence in an equalization process, for example, even when the number of symbols of a synchronization word is small.
2. Related Art
For example, in digital wireless terminal devices for land mobile telecommunications, it is desirable to use a Recursive Least Squares (RLS) algorithm in which the pull-in speed of a tap gain is fast and tap gain coefficients are sufficiently converged in 10 symbols since a propagation channel varies at fast speed when a decision feedback equalizer is introduced to compensate for distortion of a received signal due to the effect of delayed waves, particularly the digital wireless terminal devices move at fast speeds.
However, since the RLS algorithm has a problem in that the number of multiplications increases in proportion to a square of the number of taps of an adaptive filter, a computation amount significantly increases to use a complex determinant and the like in calculation, and double precision floating point computation is required, it is difficult for a fixed point Digital Signal Processor (DSP) to perform a process at low power consumption and low cost and it is difficult for the RLS algorithm to be mounted in a portable terminal wireless device.
On the other hand, a Least Mean Square (LMS) algorithm exhibits a relatively good convergence characteristic as a computation amount is reduced in proportion to the number of taps of the adaptive filter. The LMS algorithm is known as a tap gain update algorithm of a linear or non-linear equalizer.
Since the LMS algorithm controls the tap gain to gradually approach an optimal tap gain in which a mean square value of errors is minimized, at least 30 to 50 symbols are required for coefficient convergence. However, the LMS algorithm is regarded as a representative adaptive algorithm in terms of a stability level and a computation amount. To mount the LMS algorithm in a signal processor, a computation amount is reduced, a convergence time of tap gain coefficients is shortened at the time of a training operation, or the number of symbols required for the training operation is reduced.
Examples of related art are as follows:
Patent Document 1 Japanese Patent Publication No. 2004-172724;
Patent Document 2 Japanese Patent Publication No. 2004-297536;
Patent Document 3 Japanese Patent Publication No. 2003-46415;
Non-Patent Document 1 ARIB STD-T61 “Narrow-Band Digital Communication System (SCPC/FDMA),” Association of Radio Industries and Businesses; and
Non-Patent Document 2 ARIB STD-T79 “Local Digital Communication System,” Association of Radio Industries and Businesses.
As described above, the LMS algorithm whose process is simple in terms of the stability and the computation amount requires 30 to 50 symbols in a synchronization word for coefficient convergence.
However, in many mobile wireless standards such as ARIB STD-T79 (Non-Patent Document 2) and the like, a synchronization word available for training has only 10 symbols.
Now, the above problems will be described in detail.
FIGS. 4A and 4B illustrate structural examples of frames used for a conventional digital wireless communication. FIG. 4A illustrates a structural example of a frame for a control channel or a communication channel, and FIG. 4B illustrates a structural example of a frame for a synchronization burst.
One frame is constructed with symbols 41, 51 of a synchronization word sequence (SW) and information (DATA) symbols 42a, 42b, 52a, 52b. For example, the symbols 41, 51 of the synchronization word sequence (SW) are arranged at the center (or almost center) and the information (DATA) symbols 42a, 42b, 52a, 52b are arranged before and after the symbols 41, 51 of the synchronization word sequence (SW).
The synchronization word sequence (SW) 41 for the control channel or the communication channel is 10 symbols, and the synchronization word sequence (SW) 51 for the synchronization burst is 16 symbols.
Multiple frames as described above are continuously wirelessly communicated.
An embodiment as described below will be described with reference to FIG. 4. For convenience of explanation, FIG. 4 will be referred to. However, the present invention is not limited to FIG. 4.
FIG. 6A illustrates an example of a characteristic P1 of an error e(t) in the case where an equalization process is performed using a conventional LMS algorithm and an example of a characteristic P2 of an error e(t) in the case where an equalization process is performed using a conventional RLS algorithm. In a graph, the horizontal axis represents the time (symbol number) and the vertical axis represents the error e(t). In this example, the equalization process is performed using the symbols 41, 51 of the synchronization word sequence (SW) in a time band of the symbols 41, 51 of the synchronization word sequence (SW). The equalization process is performed on the basis of blind equalization (or equalization based on only information symbols) in a time band of the information (DATA) symbols 42a, 42b, 52a, 52b. 
Sufficient convergence cannot be achieved when the synchronization word is set to 10 symbols and the like in the equalization process of the communication system using the LMS algorithm requiring 30 to 50 symbols for convergence as illustrated in FIG. 6A. In this state, there is a problem in that divergence occurs when blind equalization (or equalization based on only the information symbols) is performed.
An improvement is required to perform an effective equalization process in the frame structures as illustrated in FIGS. 4A and 4B.